This invention relates to a coating process for intricate parts, and more specifically to a process for coating parts with a ceramic radar-absorbing material (RAM) by flooding a vessel containing the part with a RAM slurry.
In many military applications, there is a strong need to make aircraft, vehicles and other objects, including their component parts, as invisible to radar as possible. A number of techniques for accomplishing this purpose are well known. One such technique is to coat metallic parts with a ceramic radar-absorbing material.
Conventionally, the ceramic RAM is suspended in particulate form in a wet slurry which is sprayed onto the substrate of the part to be processed. Although this process is easy to use and is performable with readily available equipment, and has proven generally suitable for its intended purpose, it possesses inherent deficiencies which detract from its overall effectiveness and desirability. Specifically, the spray process has several disadvantages: for one, coatings of small, complex parts or parts with small internal diameters are difficult to obtain consistently; secondly, the spray process does not lend itself well to automation because variables such as coating thickness are difficult to control; and thirdly, a sprayed coating sometimes has difficulty adhering to the part with the result that electromagnetic performance is degraded.
Furthermore, because the slurry is a mixture of heavy and light particulates, it is important to maintain the slurry in a homogenous consistency. This can be done by maintaining the slurry in an agitated and/or flowing state, which keeps the heavier particles in suspension.
Other prior art methods include the following:
Nishio et al. U.S. Pat. No. 5,091,222 describes a method of ceramic coating in which the workpiece is dipped into a ceramic solution;
Van ""T Veen et al. U.S. Pat. No. 5,089,299 shows apparatus for applying a micropore coating to a ceramic substrate, in which the workpiece is moved with respect to the ceramic suspension. This is undesirable because movement of the part can disrupt the uncured coating.
Reed et al. U.S. Pat. No. 4,208,454 shows a coating process in which an alumina slurry is forced though a workpiece by a vacuum.
In view of the shortcomings of the prior art, it is desirable to provide a process which will uniformly coat parts regardless of their size or complexity, and will not be subject to the inherent inconsistencies arising from variations in spray patterns and from non-homogenity of the slurry. In this regard, although the prior art has recognized to a limited extent the nature of this problem, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy.
The present invention specifically addresses and alleviates the above mentioned deficiencies associated with the prior art. More particularly, the present invention comprises positioning the part in a vessel, and flooding the vessel with a uniformly rising level of RAM slurry. When the part has become completely submerged, the RAM slurry is drained from the vessel, and the coated part is cured or dried. The process may be repeated as often as desired to obtain a thicker coating.
In the preferred embodiments of the invention, even flooding and recycling of the RAM slurry is obtained by introducing the slurry, and also removing it, from the bottom of the vessel. The slurry is preferably stored in a variable-volume container which is preferably subjected to pressure to force the slurry into the vessel, and to a vacuum or positive-displacement device to draw the slurry out of the vessel. This method not only allows the slurry to be readily reused from one part to the next but it also allows it to be stored in a sealed, contamination-free container.
These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.